Add a new template Todo_GTD based on:

   https://www.rath.org/pages/roundup-msgfile-extension.html

   https://www.rath.org/pages/roundup-todo-template.html

This provides a good base. Update the code above to work with current roundup (was
built for 1.4.20).

Revive date handles GTD calendar.
Status waiting is used for a GTD reminder list/waiting for with revive
    to prevent it from waiting forever.

For additional GTD support, add the following from GTD 
(https://en.wikipedia.org/wiki/Getting_Things_Done, 
https://www.ionos.com/startupguide/productivity/getting-things-done-gtd/):

  new constrained links/multilink:

    context (multilink): a new keyword type starter set: @work, @home, @phone, @computer 
         This could be implemented as @work,@phone keywords instead but a separate
         multilink field may be more useful??

  implement mechanism to mark item as inbox. Inbox items need to be processed before
     becoming something that can be acted on. E.G. add "inbox" to status to find
     stuff that has been captured and needs to be processed.

  add type: project (with multiple requires), maybe/someday, next-action
       This may be a better place to put inbox pre-categorization.

Maybe add a new template for weekly review?

Also add some nice affordances:

  drag/drop upload: https://wiki.roundup-tracker.org/FileUploadViaDragDropAndPaste